The present invention relates to vector plotters and, more particularly, to an improvement for compensating for friction between the drive motors and the plotting apparatus to improve accuracy and plot quality of a vector plotter having an X-drive motor for driving plotting apparatus in +X and -X directions and a Y-drive motor for driving plotting apparatus in +Y and -Y directions and logic for applying positive and negative electrical drive currents to the respective motors to effect plotting, said improvement comprising, memory means for holding +X, -X, +Y, and -Y friction compensation factors; first supplemental logic means for, prior to the time of plotting with the vector plotter, calibrating the plotter by causing the motors to move the plotting apparatus in +X, -X, +Y, and -Y directions, measuring the friction during the +X, -X, +Y, and -Y directional movement, calculating and storing the +X, -X, +Y, and -Y friction compensation factors in the memory means; and, second supplemental logic means for, at the time of plotting with the vector plotter, adjusting the current applied to the drive motors by the plotter logic as a function of the proper friction compensation factor in the memory means for the motor moved and the direction of movement thereof.
Vector plotters are an important part of modern computer systems as employed in many environments. They provide the ability to have the computer produce large drawings for engineering uses and otherwise. While early plotters moved a pen over stationary paper in both the X and Y directions (and the present invention would have benefit in such a plotter), most contemporary plotters operate by moving the paper in the X direction and moving a pen across the paper in the Y direction. This arrangement permits the plotting of large drawings in a much smaller amount of floor space. As depicted in simplified form in FIG. 1, a typical plotter 10 has a horizontal beam 12 upon which a carriage 14 is mounted for transverse movement from side to side as indicated by the arrows 16. The carriage 14 carries the pen 18 employed to create the drawings. The carriage 14 is moved by a loop of non-stretching tape 20 passing over guide rollers 22. The tape 20 is driven by a drive system symbolized by the box 24. The drive system 24 can comprise various components (e.g. gears, belts, etc.) according to the particular implementation. The specifics of the particular components are not important and, therefore, in the interest of simplicity, are not shown. The drive system 24 is driven bi-directionally by a carriage motor 26 under the control of X--Y drive logic 28 through carriage drive input 29.
In a similar fashion, the media 30 is moved under the beam 12 (and perpendicular thereto) as indicated by the arrows 31. One popular approach, as depicted in FIG. 1, is to position a pair of drive rollers 32 opposite one another at the edges of (and gripping by pinching in combination with other rollers, not shown) the media 30. The drive rollers 32 are driven by a drive system symbolized by the box 34. Like the drive system 24, the drive system 34 can comprise various components (e.g. gears, belts, etc.) according to the particular implementation. The drive system 34 is driven bi-directionally by a media motor 36 also under the control of the X--Y drive logic 28 through media drive input 37.
In working with a new plotter of the type shown in FIG. 1 as developed by the assignee of this application, the inventors herein found that the accuracy and therefore the plot quality of the plotter was not as expected. In investigating the problem, they found nothing in the plotter's design and implementation that single-handedly accounted for the difference between expected performance and actual performance. It was eventually theorized (and ultimately proved to be true) that within the high speed environment of the plotter, the friction throughout the system (primarily in the drive systems 24 and 34), while individually relatively insignificant, was cummulatively significant with respect to the accuracy of movement accomplished by the servo motors employed therein sufficiently to degrade overall system performance. Moreover, the inherent system friction could change from plotter to plotter (probably as a result of parts tolerances), could change over time (evidently as a result of the frictional heating and expanding of parts), and could change as a result of different media and pen type combination employed in the plotting process. A graph of the performance of prior art vector plotters as a function of this drive train friction is shown in FIG. 2. As can be seen, as the friction increases, the performance (i.e., plot quality) drops off correspondingly.
Wherefore, it is the object of the present invention to provide a dynamic frictional compensation system for incorporation into vector plotters, and the like, which will permit them to achieve maximum possible positional accuracy and attendant plot quality.
It is another object of the present invention to provide a frictional compensation system for incorporation into vector plotters, and the like, which will dynamically measure, calculate, and compensate for system friction affecting positional accuracy and attendant plot quality.
Other objects and benefits of the present invention will become apparent from the description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.